In the related art, a flow-rate measurement device using a so-called sing-around method, which repeats transmission/reception between two transducers multiple times to increase measurement resolution, has been suggested.
Hereinafter, an example of a flow-rate measurement device of the related art which is applied to a household gas meter will be described with reference to FIG. 6.
FIG. 6 is a block diagram of a flow-rate measurement device using a sing-around method of the related art. As shown in FIG. 6, the flow-rate measurement device includes first transducer 42 and second transducer 43 which are provided in fluid path 41, measurement unit 44, control unit 45, and calculation unit 46. First transducer 42 which transmits (sends) an ultrasonic wave and second transducer 43 which receives the transmitted ultrasonic wave are arranged to be opposite to each other in the flow direction of a fluid flowing in fluid path 41. Measurement unit 44 measures the propagation time of an ultrasonic wave which propagates between first transducer 42 and second transducer 43. Control unit 45 controls measurement unit 44. Calculation unit 46 calculates the flow rate of the fluid flowing in the fluid path 41 on the basis of the result of the measurement of measurement unit 44.
Hereinafter, a method which calculates the flow rate of the fluid flowing in fluid path 41 will be described. As shown in FIG. 6, the sound speed is referred to as C, the flow velocity of the fluid is referred to as v, the distance between first transducer 42 and second transducer 43 is referred to as L, and the angle between the propagation direction of the ultrasonic wave and the flow direction is referred to as θ.
The propagation time when an ultrasonic wave is transmitted from first transducer 42 arranged on the upstream side of fluid path 41 and is received by second transducer 43 arranged on the downstream side is referred to as t12. The propagation time in the reverse direction of the fluid flow when an ultrasonic wave is transmitted from second transducer 43 arranged on the downstream side of fluid path 41 and is received by first transducer 42 arranged on the upstream side is referred to as t21.
At this time, the propagation time t12 and the reverse propagation time t21 are obtained by the following expressions.t12=L/(C+v cos θ)  (Expression 1)t21=L/(C−v cos θ)  (Expression 2)
Next, if (Expression 1) and (Expression 2) are modified, the flow velocity v of the fluid is obtained from (Expression 3).v=L·(1/t12−1/t21)/2 cos θ  (Expression 3)
The cross-sectional area of fluid path 41 is multiplied to the value of the fluid velocity obtained by (Expression 3) to obtain the flow rate of the fluid. At this time, the term in parentheses of (Expression 3) may be modified as (Expression 4).(t21−t12)/t12·t21  (Expression 4)
While the denominator term of (Expression 4) substantially has a constant value regardless of change in the flow velocity of the fluid, the numerator term of (Expression 4) substantially has a value proportional to the flow velocity of the fluid.
Accordingly, in order to accurately measure the flow velocity of the fluid, it is necessary to measure the difference between the propagation time t12 and the reverse propagation time t21 with satisfactory accuracy. That is, it is necessary to obtain a minute difference in the propagation time as the flow velocity of the fluid becomes lower. For this reason, when measuring the difference between the propagation time t12 and the reverse propagation time t21 singly, measurement unit 44 should have performance so as to execute a measurement, for example, with time resolution of nanosecond (ns) order.
However, it is usually difficult to realize time resolution of nanosecond (ns) order. For example, even if the time resolution of nanosecond (ns) order is realized, there is a problem in which power consumption increases due to high-speed processing.
Accordingly, in order to solve the above-described problem, usually, a flow-rate measurement device has been developed in which transmission/reception of an ultrasonic wave is first repetitively executed a plural number of times to repetitively measure the propagation time by measurement unit 44, and the average value of the propagation times measured by measurement unit 44 is obtained, thereby realizing necessary time resolution. That is, if the time resolution of measurement unit 44 is TA and the number of repetitions of the transmission/reception of the ultrasonic wave is M, measurement unit 44 is consecutively operated during a repetitive measurement, and thus the time resolution of the propagation time can be set to TA/M. Therefore, it is possible to realize the measurement of the propagation time with high accuracy when the pressure in fluid path 41 is stable.
However, when the flow-rate measurement device is applied to, for example, a gas meter which measures the flow rate of gas to be supplied as an energy source to home, there is an inherent problem which is called a pulsation phenomenon. The pulsation phenomenon is, for example, the phenomenon in which, as in an air conditioner using a gas engine which is called a GHP (Gas Heat Pump), the pressure in an ambient gas supply pipe fluctuates in synchronization with the rotation of the gas engine.
When the pulsation phenomenon occurs, even if a gas appliance is not used, gas moves in the gas supply pipe in synchronization with fluctuation in pressure. As a result, there is a problem in that the flow-rate measurement device detects the flow rate as if gas flows in the gas supply pipe.
Accordingly, as a method which suppresses the influence of the pulsation phenomenon, for example, a method described in PTL 1 has been suggested. According to the method of PTL 1, first, the number of repetitions M of the measurement is suppressed to the minimum number of times such that measurement accuracy can be maintained. Next, the number of repetition M of the measurement is set as a single measurement unit, a measurement interval is shortened, the measurement unit is executed N times consecutively for a comparatively long time little by little. The flow rate is calculated using the results of the measurement consecutively measured the N times, thereby reducing the influence of pulsation. At this time, in particular, if the measurement interval is sufficiently shorter than a pressure fluctuation period due to pulsation, it is possible to capture the phase state of the flow-velocity fluctuation waveform of the fluid evenly. The measured flow rates are averaged, thereby detecting the real flow velocity (flow rate) of the fluid with the fluctuation component due to pulsation removed.
However, when the above-described measurement method is constantly continued, while the influence of pulsation can be reduced, there is a problem in that power consumption increases.
Accordingly, in order to solve the above-described problem, for example, a method described in PTL 2 has been suggested. According to the method of PTL 2, in order to reduce power consumption, the number of measurements N is controlled in accordance with fluctuation in the flow rate of the fluid. Specifically, a measurement method has been suggested, in which, when the fluctuation in the flow rate of the fluid is small and it can be determined that there is no pulsation, the number of measurements N is reduced, and when fluctuation in the flow rate of the fluid is large and there is pulsation, the number of measurements N increases.
In the configuration of PTL 2, however, when pulsation does not occur, while power consumption can be reduced, there is no disclosure about a measurement method which reduces power consumption according to the magnitude of the flow rate of the fluid.
That is, for example, as in a gas meter which has a battery or the like as a drive source, in order to effectively use a limited amount of power resources, first, when there is no pulsation, power consumption is suppressed. When there is no influence on an integrated flow rate, that is, there is no fluid flow, there is a need for a method which suppresses the frequency of the flow-rate measurement operation, thereby reducing power consumption of the entire flow-rate measurement device.    PTL 1: Japanese Patent Unexamined Publication No. 2002-350202    PTL 2: Japanese Patent No. 3427839